1. Field of the Invention
The present invention relates to circuits and other apparatus and to methods for tone decoding. More particularly, the present invention relates to apparatus and methods for decoding single tones and dual tones, such as those used in telephone applications to decode DTMF tones, dial tones, busy signals and other tones used in telephone systems, for recognizing call progress signals in PBX and other telephone systems, for adapting tone recognition systems to new environments, and for recognizing voice energy.
2. The Prior Art
Modern telephone systems automate and implement many useful functions by detecting the presence of control tones and response tones generated by the telephone company central office or local PBX in response to the condition of individual telephone sets in the system. Depending on location, telephone systems throughout the world utilize different combinations of periodic or continuous single tones and dual tones, such as dial tones, DTMF tones, ringback and no-ringback signals, busy signals, and connect signals.
Circuits are known in the prior art for detection of single and dual tones used in telephone systems, and for classifying these tones by their function. A combination of frequency decoding and cadence detection is commonly employed to identify telephone system tones. Frequency decoding, as its name suggests, identifies tones by their characteristic frequency or frequencies. Cadence detection looks for a repeating pattern in the audio signal, such as the pattern produced by a ringback or a busy signal. These patterns are called audio cadences. Once a cadence has been established, it can be classified as a single ring, a double ring, or a busy signal by comparing the periods of silence and sound to established parameters stored in or otherwise known by the circuitry. Cadence detection differentiates between the no-ringback, no-answer, busy, and connect states of a telephone system. An example of a prior art cadence detection system is found in U.S. Pat. No. 4,477,698.
The capability of a signal processing system to identify these different call progression tones and to differentiate between these tones, noise, and voice energy on a signal line would be useful capability. It is therefore an object of the invention to provide apparatus and a method which is capable of identify different call progression tones and differentiating between these tones, noise, and voice energy on a signal line.